In many oil fields the oil bearing formation comprises a gas cap zone and an oil bearing zone. Many of these fields produce a mixture of oil and gas with the gas to oil ratio (GOR) increasing as the field ages. This is a result of many factors well known to those skilled in the art. Typically the mixture of gas and oil is separated into an oil portion and a gas portion at the surface. The gas portion may be marketed as a natural gas product, injected to maintain pressure in the gas cap or the like. Further, many such fields are located in parts of the world where it is difficult to economically move the gas to market therefore the injection of the gas preserves its availability as a resource in the future as well as maintaining pressure in the gas cap.
Wells in such fields may produce mixtures having a GOR of over 10,000 standard cubic feet per standard barrel (SCF/STB). In such instances, the mixture may be less than 1% liquids by volume in the well. Typically a GOR from 800 to 2,500 SCF/STB is more than sufficient to carry the oil to the surface as a gas/oil mixture. Normally the oil is dispersed as finely divided droplets or a mist in the gas so produced. In many such wells quantities of water may be recovered with the oil. The term "oil" as used herein refers to hydrocarbon liquids produced from a formation. The surface facilities for separating and returning the gas to the gas cap obviously must be of substantial capacity when such mixtures are produced to return sufficient gas to the gas cap or other depleted formations to maintain oil production.
Typically, in such fields, gathering lines gather the fluids into common lines which are then passed to production facilities or the like where crude oil, condensate, and other hydrocarbon liquids are separated and transported as crude oil. Natural gas liquids are then recovered from the gas stream and optionally combined with the crude oil and condensate. Optionally, a miscible solvent which comprises carbon dioxide, nitrogen and a mixture of light hydrocarbons such as contained in the gas stream may be used for enhanced oil recovery or the like. The remaining gas stream is then passed to a compressor where it is compressed for injection. The compressed gas is injected through injection wells, an annular section of a production well, or the like, into the gas cap.
Clearly the size of the surface equipment required to process the mixture of gas and oil is considerable and may become a limiting factor on the amount of oil which can be produced from the formation because of capacity limitations on the ability to handle the produced gas.
It has been disclosed in U.S. Pat. No. 5,431,228 "Down Hole Gas-Liquid Separator for Wells" issued Jul. 11, 1995 to Weingarten et al and assigned to Atlantic Richfield Company that an auger separator can be used downhole to separate a gas and liquid stream for separate recovery at the surface. A gaseous portion of the stream is recovered through an annular space in the well with the liquids being recovered through a production tubing.
In SPE 30637 "New Design for Compact Liquid-Gas Partial Separation: Down Hole and Surface Installations for Artificial Lift Applications" by Weingarten et al it is disclosed that auger separators as disclosed in U.S. Pat. No. 5,431,228 can be used for downhole and surface installations for gas/liquid separation. While such separations are particularly useful as discussed for artificial or gas lift applications and the like, all of the gas and liquid is still recovered at the surface for processing as disclosed. Accordingly, the surface equipment for processing gas may still impose a significant limitation on the quantity of oil which can be produced from a subterranean formation which produces oil as a mixture of gas and liquids.
A co-pending patent application, Ser. No. 09/028,624, entitled "Method and System for Separating and Injecting Gas in a Wellbore", filed Feb. 24, 1998, discloses driving a turbine with production fluids, separating gas from an oil-enriched mixture of oil and gas, compressing the gas with a compressor driven by the turbine, and injecting the gas back into the formation. While such a system relieves the load on surface processing equipment, the blades in the turbine are vulnerable to damage from solids and liquids produced in some formations and therefore may require periodic maintenance to cure such damage.
Accordingly, a continuing search has been directed to the development of systems which permit increased amounts of oil to be produced from subterranean formations which produce, in addition to oil and gas, liquids and/or solids which may damage turbine blades.